High elevation shrubification in the subarctic during the Holocene Thermal Maximum and 21st century

Future changes in high latitude shrubification are expected to amplify warming. However, the elevational range and climate sensitivity of shrubification is poorly constrained, which hinders our ability to tune Earth system models. Using a sediment record from a high-elevation lake in Iceland (422 m asl), we leverage ancient birch DNA and mean summer air temperature reconstructed from bacterial lipids to constrain the climate sensitivity of highland shrubification. Our data show that during the Holocene Thermal Maximum, summer temperatures were 2.3 oC warmer than present and birch was present in the lake catchment, >150 m higher than its current regional limit. Subsequently, climate remained the dominant Holocene control on Icelandic birch cover at this site, even after human settlement – making our data ideal constraints for Earth system models. Using Holocene Thermal Maximum warmth as a partial analog for future climate change, ensemble climate projections suggest that the re-expansion of birch to higher elevations is likely by 2100 CE. Birch woodlands in Iceland were hundreds of meters higher during past warm periods in Earth history than at present, according to complementary lake sediment records of ancient plant DNA and lipid-inferred temperatures.